Surface optical phonon—assisted electron Raman scattering in a semiconductor quantum disc

We have carried out a theoretical calculation of the differential cross section for the electron Raman scattering process associated with the surface optical phonon modes in a semiconductor quantum disc.electron states are considered to be confined within a quantum disc with infinite potential barriers.The optical phonon modes we have adopted are the slab phonon modes by taking into consideration the Frohlich interaction between an electron and a phonon.The selection rules for the Raman process are given.Numerical results and a discussion are also presented for various radii and thicknesses of the disc,and different incident radiation energies.     

Electronic Raman scattering and the electron–phonon interaction in YB6

Electronic Raman scattering in YB₆ and in its structural and electronic analog LaB₆ has been studied in the temperature range of 10–730 K. The experimental spectra have been compared to those calculated on the basis of ab initio band structures with renormalization owing to the electron–phonon interaction. Good agreement between the calculation and experiment for LaB₆ has been obtained throughout the entire temperature range. This allows the determination of the coupling constant λ _ep = 0.25. To satisfactorily describe the spectra of electronic light scattering in YB₆, it is necessary to introduce an additional electron relaxation channel. In this case, the estimate of the electron–phonon coupling constant λ _ep is no more than 0.4; for this reason, a high superconducting transition temperature cannot be explained only by the phonon mechanism.     

Pseudotime Schrödinger equation with absorbing potential for quantum scattering calculations

The Schr?dinger equation (Hpsi) (r) = [E+u(E)W(r)]psi(r) with an energy-dependent complex absorbing potential -u(E)W(r), associated with a scattering system, can be reduced for a special choice of u(E) to a harmonic inversion problem of a discrete pseudotime correlation function y(t) = phi(T)U(t)phi. An efficient formula for Green's function matrix elements is also derived. Since the exact propagation up to time 2t can be done with only approximately t real matrix-vector products, this gives an unprecedently efficient scheme for accurate calculations of quantum spectra for possibly very large systems.     

Tailoring electron–phonon interaction in nanostructures

Efficient design of optoelectronic devices based on electron intersubband transitions depends critically on the knowledge of the intersubband relaxation times which in turn, depends on electron scattering with LO and acoustic phonons. In this article the intersubband scattering time associated with electron–acoustic-phonon interaction has been discussed in terms of phonon mode quantization and phonon confinement with describing the acoustic phonon dispersion relation in detail by introducing the cut-off frequency for each mode. It has been shown that the quantization of acoustic phonon modes lead to an enhancement in electron–phonon scattering time in AlGaAs quantum well structures. Based on the presented model, a new tailoring method has presented to adjust the electron–phonon scattering time in intersubband-transition-based structures while keeping the electronic properties unaltered. Also, we illustrated that for a quantum well with subband energy separation of ∼30 meV, the intersubband scattering time with acoustic-phonon-assisted transitions could be tailored from ∼120 ps to increased value of ∼400 ps or reduced value of ∼45 ps by inserting a 1 nm-thickacoustically soft or hard layers, respectively, while keeping the same the initial energy separation.     

Oscillatory electron phonon coupling in Pb/Si（111）deduced by temperature-dependent quantum well states

Photoemission study of atomically flat Pb films with a thickness from 15 to 24 monolayers （ML） have been performed within a temperature range 75-270K. Well-defined quantum well states （QWSs） are observed, which exhibit interesting temperature-dependent behaviours. The peak position of the QWSs shifts towards higher binding energy with increasing substrate temperature, whereas the peak width broadens linearly due to enhanced electron-phonon coupling strength （λ）. An oscillatory A with a period of 2ML is deduced. Preliminary analysis shows that the oscillation can be explained in terms of the interface induced phase variations, and is thus a manifestation of the quantum size effects.     

Studies of high-temperature electron–phonon interactions with inelastic neutron scattering and first-principles computations

Several recent studies of phonons combining inelastic neutron scattering and first-principles calculations are summarized. Inelastic neutron scattering was used to measure the phonon densities of states of the A15 compounds V₃Si, V₃Ge, and V₃Co at temperatures from 10 K to 1273 K. It was found that phonons in V₃Si and V₃Ge, which are superconducting at low temperatures, exhibit an anomalous stiffening with increasing temperature, whereas phonons in V₃Co have a normal softening behavior. Additional measurements of the phonon DOS of BCC V alloys were performed, and it was found that a stiffening anomaly present in pure V is suppressed upon introduction of extra d-electrons by alloying. First-principles calculations of the electronic and phonon densities of states show that in both these systems, the anomalous phonon stiffening originates with an adiabatic electron–phonon coupling mechanism. The anomaly is caused by the thermally-induced broadening of sharp peaks in the electronic density of states, which tends to decrease the electronic density at the Fermi level. These results illustrate how the combined use of first-principles calculations and inelastic neutron scattering provides powerful insights into couplings of excitations in condensed-matter.     

Electron–phonon interaction in fan-shaped quantum dot and quantum wire

The optical phonon modes and electron–optical-phonon interaction in fan-shaped quantum dot and quantum wire are studied with the dielectric continuum (DC) model and separation of variables. The explicit expressions for the longitudinal optical (LO) and interface optical (IO) phonon eigenmodes are deduced. It is found that there exist two types of IO phonon modes: top interface optical (TIO) phonon mode and arc interface optical (AIO) phonon mode, in a fan-shaped quantum dot. After having quantized the eigenmodes, we derive the Hamiltonian operators describing the LO and IO phonon modes as well as the corresponding Fröhlich electron–phonon interaction. The potential applications of these results are also discussed.     

Gate-controlled electron–electron interactions in an In0.53Ga0.47As/InP quantum well structure

We study the parabolic negative magnetoresistivity in a gated In_0.53Ga_0.47As/InP quantum well structure where the scattering potential is predominantly long range. This magnetoresistivity is caused by the electron–electron interactions and is fitted to estimate the interaction corrections to the Drude conductivity. These corrections are smaller than the prediction of a recent theory [I.V. Gornyi, A.D. Mirlin, Phys. Rev. Lett. 90 (2003) 076801], and can be quantitatively described by Altshuler’s theory.     

Effects of electron–electron scattering in wide ballistic microcontacts

We consider effects of electron–electron scattering in wide ballistic microcontacts. Using a semiclassical Boltzmann equation, we obtain a positive correction to the Sharvin conductance that results from electron–electron collisions in the leads. The correction is linearly dependent on temperature at high temperatures T?eV$T?\mathit{eV}$ and proportional to |V|$\left|V\right|$ at high voltages eV?T$\mathit{eV}?T$. Magnetic field leads to strong suppression of this positive correction that results in a positive magnetoresistance in weak fields. As electron–electron scattering affects the conductance, it also influences the noise. At low voltages the noise is defined by the Nyquist relation and at high voltages it is related with the inelastic correction to the current by the Shottky formula δS=2eδI$\delta S=2e\delta I$.     

Optical phonon self-energies in titanium phases: effects of electron–phonon interaction

Temperature-dependent Raman investigations of titanium in the α and pressure-quenched ω-phase have been carried out. The results obtained suggest the possible coexistence of both phases at ambient pressure and low temperatures. Comparison of the low-temperature E_2g phonon self-energies in both phases with simulations based on the calculated electronic structures for α- and ω-Ti implies significant contributions from non-adiabatic electron–phonon interactions.     

Photoluminescence properties of exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We report on the photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well under high-density-excitation conditions at excitation energies near the fundamental exciton energies. The biexciton-PL band is dominant in a relatively low-excitation-power region. The PL originating from exciton–exciton scattering, the so-called P emission, suddenly appears with an increase in excitation power. The excitation-energy dependence of the intensity of the P-PL band indicates that the excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical (LO) phonon is the most efficient for the P PL. This suggests that the LO-phonon scattering plays an important role in the relaxation process of excitons leading to the P PL. The appearance of the P-PL band remarkably suppresses the intensity of the biexciton-PL band; namely, the exciton–exciton scattering process prevents the formation of biexcitons. Furthermore, we have confirmed the existence of optical gain due to the exciton–exciton scattering process with use of a variable-stripe-length method.     

Photoluminescence due to exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We have investigated photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well at 10 K under intense excitation conditions. It has been found that a PL band due to exciton–exciton scattering, the so-called P emission, is observed in addition to the biexciton PL under an excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical phonon. On the other hand, the P band could never be observed at an excitation energy much higher than the exciton energy, where a band-filling phenomenon appears in the PL spectrum. Furthermore, we confirmed the existence of optical gain leading to stimulated emission in the energy region of the P band using a variable-stripe-length method.     

Spin–orbit scattering effect on electron–electron interactions in disordered metals

We have measured the low-temperature resistivities of a series of bulk crystalline disordered Ti_73−xAl₂₇Sn_x alloys (x≲5) as well as the sheet resistances of a number of thin ferromagnetic Ni films (≈120 Å thick) sandwiching an ultrathin Ag or Au (≲5 Å) layer. The level of impurities (concentration of Sn in the former case, and thickness of Ag or Au in the latter case) is progressively increased in order to enhance the spin–orbit scattering in a controllable manner. The influence of the spin–orbit scattering on the electron–electron interaction effects is studied from the temperature dependence of resistivities (sheet resistance) at low temperatures. We find that the electron–electron interaction contribution to the resistivities (sheet resistances) increases slightly with increasing spin–orbit scattering. Our observation is discussed in terms of the current theoretical concept for the electron–electron interactions in disordered metals.     

Effects of electron–optical phonon interactions on the polaron energy in a wurtzite ZnO/Mg_xZn_(1-x)O quantum well

We investigated the properties of polarons in a wurtzite ZnO/MgxZn1-xO quantum well by adopting a modified Lee–Low–Pines variational method, giving the ground state energy, transition energy, and phonon contributions from various optical-phonon modes to the ground state energy as functions of the well width and Mg composition. In our calculations, we considered the effects of confined optical phonon modes, interface-optical phonon modes, and half-space phonon modes, as well as the anisotropy of the electron effective band mass, phonon frequency, and dielectric constant. Our numerical results indicate that the electron–optical phonon interactions importantly affect the polaronic energies in the ZnO/MgxZn1-xO quantum well. The electron–optical phonon interactions decrease the polaron energies. For quantum wells with narrower wells, the interface optical phonon and half-space phonon modes contribute more to the polaronic energies than the confined phonon modes. However, for wider quantum wells, the total contribution to the polaronic energy mainly comes from the confined modes. The contributions of the various phonon modes to the transition energy change differently with increasing well width. The contribution of the half-space phonons decreases slowly as the QW width increases, whereas the contributions of the confined and interface phonons reach a maximum at d ≈ 5.0 nm and then decrease slowly. However,the total contribution of phonon modes to the transition energy is negative and increases gradually with the QW width of d.As the composition x increases, the total contribution of phonons to the ground state energies increases slowly, but the total contributions of phonons to the transition energies decrease gradually. We analyze the physical reasons for these behaviors in detail.     

Anomalous spin–orbit coupling in high-density two-dimensional electron gas confined in InGaAs/InAlAs quantum well

We study the magnetotransport property of a high-density two-dimensional electron gas confined in InGaAs/InAlAs quantum well. Both beating pattern in the Shubnikov–de Hass oscillation of resistivity and weak antilocalization effect are observed. From these two effects, Rashba spin-splitting energy is extracted. The extracted Rashba spin-splitting energy shows a nonmonotonic dependence on Fermi wave vector, contrary to the prevailing linear Rashba model. This anomalous behavior can be attributed to the nonlinear Rashba spin-splitting mechanism [Yang et al., Phys. Rev. B 74 (2006) 193314].     

Formation of excitons from free electron–hole pairs due to acoustic phonon interactions in quantum wells

A study of the process of exciton formation due to acoustic phonon interaction in quantum wells (QWs) is presented. Considering that excitons are formed from photoexcited free electron–hole pairs, we have derived the rate of such formation as a function of density and temperature of charge carriers and wavevectorK_|| of the center-of-mass motion of exciton, and finally applied our theory to GaAs/AlGaAs QWs. We have found that the formation of an exciton due to acoustic phonon emission is more efficient at relatively large values ofK_|| (hot excitons) whereas that due to longitudinal optical (LO) phonon emission is more efficient at relatively small values of K_||.     

Phonon focusing and electron–phonon drag in semiconductor crystals with degenerate charge-carrier statistics

We study the effect of anisotropy in elastic properties on the electron–phonon drag and thermoelectric phenomena in gapless semiconductors with degenerate charge-carrier statistics. It is shown that phonon focusing leads to a number of new effects in the drag thermopower at low temperatures, when diffusive phonon scattering from the boundaries is the predominant relaxation mechanism. We analyze the effect of phonon focusing on the dependences of the thermoelectromotive force (thermopower) in HgSe:Fe crystals on geometric parameters and the heat-flow directions relative to the crystal axes in the Knudsen regime of the phonon gas flow. The crystallographic directions that ensure the maximum and minimum values of the thermopower are determined and the role of quasi-longitudinal and quasi-transverse phonons in the drag thermopower in HgSe:Fe crystals at low temperatures is analyzed. It is shown that the main contribution to the drag thermopower comes from slow quasi-transverse phonons in the directions of focusing in long samples.